feat: dotnet sln migrate

[nmummau]

Description

I noticed the repo was still using .sln, so I am contributing simply to modernize the repo to a .slnx.

• I ran dotnet sln migrate to convert the solution to a .slnx.
• I deleted the .sln file.

Code Changes

• Unit tests are added, if possible
• Existing tests are passing
• New or updated code follows the guidelines here
• Ensure .dacpac from changes to Microsoft.Build.Sql build process MUST be backwards compatible with older versions of SqlPackage
• Use proper logging for MSBuild tasks

[nmummau]

@microsoft-github-policy-service agree